Is he pushing the defender out of his way?

He is not allowed to displace the defender. It's no different than taking 2 hands and shoving the defender backwards.

And it applies to rebounding also. You here folks complain "I/He was just boxing out".

There is a difference between blocking out and backing out. If the A1 uses his body to shield or hold his position but is not displacing his player he is legal, if he uses his body to back B1 out of position (usually by several feet) this is illegal.

In reference to the original question, always look for first contact in the post. In the OP situation, I agree with others who say an arm bar may be in reaction to the other play pushing them back so if you look for and get first contact (A1 moves back causing B1 to put up arm bar or B1 leans in with arm bar but A1 stays in original position) then it will fix itself most of the time.

First contact is also key to officiating rough post play when tow players are doing a lot of pushing underneath to gain position. Get it early and it will usually stop.

If it is collapsed, or only used as a cushion, that is, neither extended nor horizontal, 4-24-3 applies.
4-24-5 and 6 are the citations that refer to what we usually consider an armbar.

[QUOTE=bob jenkins;926802] "If the defense is displaced, then it's a foul on the offense. If the defense gives way, then it's legal. Deciding which is which is wny we get paid the big bucks."

This is the essence of an official's judgement in such situations.